1. Field of the Invention
The present invention relates to a digital image signal encoding device, a digital image signal decoding device, a digital image signal encoding method, and a digital image signal decoding method used for an image compression encoding technology or a compressed image data transmission technology.
2. Description of the Related Art
An international standard video encoding system such as MPEG or ITU-TH. 26x (e.g., “Information Technology Coding of Audio-Visual Objects Part 10: Advanced Video Coding,” ISO/IEC 14496-10, 2003 (hereinafter, referred to as Non-Patent Document 1)) has conventionally been premised on use of a standardized input signal format called a 4:2:0 format. The 4:2:0 format is a format where a color moving image signal of RGB or the like is converted into a luminance component (Y) and two chrominance components (Cb, Cr), and the number of chrominance component samples is reduced to half of luminance components both in horizontal and vertical directions. The chrominance component is inferior to the luminance component in visibility. Accordingly, the conventional international standard video encoding system has been based on the premise that the amount of original information to be encoded is reduced by downsampling chrominance components before encoding is executed as mentioned above. On the other hand, recent increases in resolution and gradation of a video display have been accompanied by studies on a system for performing encoding by maintaining the number of samples equal to that of luminance components without downsampling chrominance components. A format where the numbers of luminance and chrominance component samples are completely equal is called a 4:4:4 format. The conventional 4:2:0 format has been limited to Y, Cb, and Cr color space definitions because of the premise of downsampling of chrominance components. In the case of the 4:4:4 format, however, because there is no sample ratio distinction between color components, R, G, and B can be directly used in addition to Y, Cb, and Cr, and a plurality of color space definitions can be used. An example of a video encoding system targeting the 4:4:4 format is, Woo-Shik Kim, Dae-Sung Cho, and Hyun Mun Kim, “INTER-PLANE PREDICTION FOR RGB VIDEO CODING,” ICIP 2004, October 2004 (hereinafter, referred to as Non-Patent Document 2). Non-Patent Document 2 proposes an approach of reducing a size of data which is to be encoded by performing prediction between different color components by employing a correlation remaining between the color components. However, a degree of the correlation between the color components varies depending on types of video contents and color spaces, and the prediction might adversely affects in terms of encoding efficiency. Further, the signal processing is required over a plurality of color components so such a problem arises in that parallel processing efficiency deteriorates in, for example, executing a real time processing of video signals having an extremely high resolution as a digital cinema picture (having 4000×2000 pixels).